The present invention relates to blow-molded foamed films of polyester resin and to the method for producing them.
Polyester resin films, particularly biaxially-stretched ones, are widely used in various technological fields by virtue of their high-level properties in mechanical and electrical terms as well as in terms of resistance to chemicals.
Biaxially-stretched films are superior to other films both in their tensile properties, particularly their high elastic modulus, and in their dimensional stability.
However, the films have drawbacks, mainly due to their high relative density and to the lack of opacity, which for some applications, for example in the field of computer technology and photography, require the films to be rendered opaque by using large amounts of white pigments.
Foamed mono- or biaxially-stretched films of polyester resin allow to solve the problems of non-foamed films and at the same time allow to have entirely new properties with respect to non-foamed films, such as for example high impact strength and considerable heat-shrinking.
Mono- and biaxially-stretched foamed films of polyester resin are disclosed in a previous patent application in the name of the Applicant (European application 991220468).
The films are prepared by extruding, under foaming conditions (extrusion-foaming), a polyester resin having Theological characteristics suitable for forming foamed cellular products, and by then subjecting the foamed sheets or films to mono- or biaxial stretching, using stretch ratios which can be as high as 5:1.
It has now been found unexpectedly that it is possible to obtain, from polyester resins having specific Theological characteristics which are suitable for forming foamed materials, blown foamed films (not orientated or biaxially orientated) by blow-molding a foamed tubular sheet of polyester resin at the outlet of the foaming extruder, using the single- or double-bubble method and so as to respectively obtain stretched films which are substantially not orientated, and biaxially orientated stretched films.
The polyester resin suitable for the process according to the invention has a melt viscosity of more than 2000 Pa.s at 280xc2x0 C. with a shear rate which tends to zero and a melt strength of more than 10 cN at 280xc2x0 C.
Preferred melt viscosity values are between 5000 and 20000 Pa.s; the melt viscosity is preferably higher than 50 cN and can reach 150 cN or more.
Intrinsic viscosity is generally between 0.8 and 1.5 dl/g.
Polyester resins having the above indicated Theological properties are preferably obtainable by solid-state polycondensation (SSP) of a polyester resin which has an intrinsic viscosity generally lower than 0.7-0.8 dl/g, with the addition of a dianhydride of a tetracarboxylic aromatic acid, representatively pyromellitic dianhydride, in amounts from 0.05 to 2% by weight, working under temperature conditions (generally between 80 and 220xc2x0 C.) and with durations which bring the melt strength and melt viscosity to the intended values.
The solid-state polycondensation is performed according to known methods, such as for example the method described in U.S. Pat. No. 5,243,000, whose description is hereby incorporated by reference.
Other suitable methods are described in U.S. Pat. Nos. 5,288,764 and 5,229,432, whose description also is incorporated herein by reference.
The starting polyester resins that can be used to obtain the above described Theological properties are prepared by polycondensation, according to known methods, of dicarboxylic aromatic acids with diols with 2-12 carbon atoms or by transesterification of lower alkyl esters of dicarboxylic acids with diols with 2-12 carbon atoms and subsequent polycondensation of the diol esters.
Preferred aromatic acids are terephthalic acid, isophthalic acid and naphthalene dicarboxylic acids.
Preferred resins are polyethylene terephthalate and its copolymers in which from 1 to 20-25, preferably 2-15, units derived from terephthalic acid are replaced with units derived from isophthalic and/or naphthalene dicarboxylic acids.
The polyester resins can be used in mixture with other resins, particularly polyamides, used in amount from 2 to 50% by weight on the polyester resin.
A polyamide which is particularly suitable especially to give improved gas barrier properties (against oxygen and CO2) is poly-m.xylylidene adipamide.
This polyamide is mixed in the molten state with the polyester resin, which is premixed, likewise in the molten state, with a dianhydride of a tetracarboxylic aromatic acid, preferably pyromellitic dianhydride, in an amount from 0.05 to 2% by weight on the polyester resin.
The addition of the polyamide resin (which can be aliphatic or aromatic, with high or low average molecular weight) also allows to considerably reduce the content of unreacted dianhydride in the foamed film.
Other resins which can be added to the aromatic polyester resin are aliphatic polyester resins obtainable from aliphatic dicarboxylic acids with 4-22 carbon atoms, such as adipic acid, and from aliphatic diols with 2-22 carbon atoms, or from aliphatic hydroxy-acids or from the corresponding lactones or lactides.
Poly-epsilon-caprolactone is one of the preferred resins.
The aliphatic polyester resins (which are highly biodegradable) are added in order to give the aromatic polyester resin biodegradability or biocompostability characteristics.
The foamed tubular sheet suitable for preparing the blown films is extruded from annular heads with a slit generally between 0.1 and 0.5 mm.
The amount of foaming agent (nitrogen, carbon dioxide or liquid hydrocarbons or other agents) is such as to allow to obtain apparent densities of the non-blow-molded foamed sheet between approximately 50 and 700 kg/m3.
The extrusion-foaming process for forming the foamed sheet is performed according to known methods.
Similarly, the blow-molding of the tubular sheet is performed according to known methods, using the single- or double-bubble method depending on whether one wishes to produce non-oriented or bi-oriented blow-molded films.
The tensile properties of the bi-oriented blow-molded films are far superior to those of non-oriented films.
A common characteristic of both film types is their great heat-shrinkage, which makes them particularly suitable for applications such as labeling and packaging of palletized units.
Another characteristic which is particularly evident in the bi-oriented films is their high birefringence, which makes them particularly suitable for olographic printing and similar applications.
The good bonding of inks of blow-molded films makes them particularly suitable in applications as paper replacement.
The biocompostability of the films when they are prepared from compositions comprising a biodegradable aliphatic polyester, to which starch can optionally be added, makes them suitable for producing compostable bags for waste collection.
In preparing the film, the ratio between the extrusion rate of the tubular sheet and the winding rate of the blow-molded film is higher than 1 and is generally between 2 and 8; the ratio between the diameter of the extrusion head and the diameter of the bubble is greater than 2 and is generally between 4 and 6.
The density of the blow-molded films is generally from 0.05 to 0.5 cm3.
The thickness of the films is generally from 10 to 100 microns.
Intrinsic viscosity was measured in a 60/40 by weight solution of phenol and tetrachloroethane at 25xc2x0 C. according to ASTM D 4603-86.
Rheological measurements were made according to the ASTM D 3835, using a Goettfert rheometer at 280xc2x0 C.
Melt strength was determined by measuring the force in cN (centinewtons) required to stretch the material extruded from the capillary of a Goettfert Rheograph 2002 rheometer.
For measurement, a Rheotens unit was applied at the outlet of the capillary of a Goettfert Rheograph 2002 rheometer.
The extrusion conditions are as follows:
plunger speed: 0.2 mm/sec
die diameter: 2 mm
capillary length: 30 mm
test temperature: 280xc2x0 C.
Measurements are performed by setting an acceleration of 2.4 cm/sec2.
Each measurement is repeated and the average of the two measurements taken.